Patients admitted in the intensive care unit presenting ARDS criteria persistent for 12 hours to 24 hours, will be randomly assigned to the treatment by Depelestat or placebo, after a pre-treatment period as short as possible with a maximum of 48 hours since ARDS criteria are met. During the pre-treatment period, the informed consent will be obtained from the legally authorized relative of the patient, and pre-treatment biological and functional examinations will be performed, particularly blood samples and BAL for biological parameters measurement, and ventilatory mechanics, for assessment of static compliance of the respiratory system.